Explainer: Glossary of terms

Some terms and materials relevant to FLEET.

The first sentence or two of each definition is aimed at non-physicists. For physicists in the field, these are suggested as accurate, non-jargon simplifications to help communicate your science. More information for experts is given in italics for some terms, as well as useful links. 

2D Two-dimensional materials are a single layer of atoms in thickness. 2D materials are key to FLEET research themes 1 and 2 (see below), while 2D atomic gases are used in research theme 3. The most well-known such material is graphene.

ARPES technique used to study electronic properties of materials (eg, at Tsinghua University and now at Australian Synchrotron). Read about ARPES here. Stands for angle-resolved photoemission spectroscopy.

artificial topological systems Artificial analogues of graphene and 2D topological insulators, achieved via advanced nanofabrication techniques combined with conventional semiconductors.

atomically thin materials Materials that one or a few layers of atoms in thickness (the term is often used interchangeably with 2D materials)

bandgap The energy gap (between valence electrons and conduction electrons) that defines whether a material is a conductor (no gap), insulator (large gap) or semiconductor (small gap). Semiconductor physics is full of references to gap ‘size’, for example a large bandgap is required for a material to still be topological at room temperature. (If there is sufficient energy available at room temperature to ‘bridge’ that bandgap, then your semiconductor becomes a conductor, and can no longer ‘switch’.)

band theory The theory that explains why some materials conduct electricity and others are elements. See Michael Fuhrer’s maths-free explanation.

bilayer exciton transistor Proposed transistor with twin layers of material carrying electrons in one layer and ‘holes’ in the bottom layer pair, forming strongly-bound excitons that would flow as a superfluid with very low resistance. Bilayer (or twin-layer) exciton transistors would form logic circuits just as silicon-based transistors do now.

Bose-Einstein condensates An ultra-low temperature quantum state in which all particles have the same energy and wavelength. Studied in FLEET’s research themes 2 and 3. See FLEET Advisor Wolfgang Ketterle’s talk, press release and new BEC equipment at Monash Uni.

chiral/chirality This term refers to symmetry. A molecule or system is ‘chiral’ if it is perfectly symmetrical; it’s own mirror image. (Also sometimes referred to a ‘handedness’, as in left-hand versus right hand.) Chiral spin states such as skyrmions and chiral domain walls are sought as potentially useful building blocks for topological spintronic devices which can be driven by ultra-low current density.

CMOS traditional silicon-based complementary metal oxide semiconductor (CMOS) electronics are forecast to hit their physical limits around the end of the current decade, with power consumption a significant factor limiting their performance; qualitatively new technologies are needed to extend the IT revolution.

Dirac materials A class of materials that includes graphene and topological insulators, Dirac semimetals, Weyl semimetals, some high-temperature superconductors and liquid He3.

dissipationless current Electric current that flows without wasted dissipation of energy. Power consumption in electronics occurs because there is a resistance to the flow of electrical current. The larger the resistance, the
more power is dissipated. FLEET has identified three approaches that could realistically lead to dissipationless electronic conduction at room temperature.: topological phases, superfluidity, and non-equilibrium states of matter

domain walls Atomically-sharp topological defects separating regions of uniform polarisation in ferroelectric materials. Domain walls are electrically conductive, while the surrounding of the wall (ie, the bulk of the material) is insulating. Studied at UNSW. The basis of ‘nanoelectronics’, in which the wall (rather than the bulk of the material) stores binary data.

electronic smoothness Free of electronic imperfections(see recent paper looking at electronic smoothness in Na₃Bi).

equilibrium state The state in which a material is in balance, unchanging with time (see also non-equilibrium state)

exciton Quasi-particle formed of two strongly-bound charged particles: an electron and a ‘hole’. Exciton superfluids are studied in FLEET’s research theme 2.

exciton–polariton Part matter and part light quasi-particle: an exciton bound to a photon. Exciton–polariton superfluids are studied in FLEET’s research theme 2.

exfoliating bulk single crystal A method used to create novel 2D materials for FLEET research

ferroelectric materials can be considered an electrical analogy to ferromagnetic materials, with their permanent electric polarisation resembling the north and south poles of a magnet.

ferromagnetic materials  Material that can be magnetised. See study of ferromagnetic semiconductors at UoW. These are materials in which electron spin can be aligned to form a strong magnetic field (this is what it means that they can be ‘magnetised’). Ferromagnetism is a phenomenon where unpaired electron spins are lined up parallel to each other, resulting in an intense magnetic field, within a region of the bulk material. When a small magnetic field is applied, all the spins are 100% aligned, or parallel to, the field direction. Also see paramagnetic materials.

focussed ion beam (FIB) A microscope that uses a tight beam of ions to study nanoscale structures, and can also deposit or remove materials to form new structures.

Floquet topological insulator A topological insulator created by applying light to a conventional insulator, studied within FLEET’s research theme 1.

glove box Sealed container allowing manipulation within a controlled atmosphere via gloves. Hilarious when pressure is reversed.

graphene a single 2D layer of carbon atoms. Graphene is the most well-known 2D material, and is famous for it’s remarkably high electron mobility, however it has no bandgap, and thus cannot be ‘switched’ as in a transistor. Prone to charge puddling, and thus often combined with h-BN to form electronically smooth material. Read about graphene-based bio sensors at Monash Uni. The first quantitative measurements of graphene resistivity/conductivity were done by FLEET’s Michael Fuhrer.

heterostructure A structure in which two dissimilar materials are brought together at a controlled interface

holes The absence of an electron. Also see excitons. Read UNSW study regarding hole spin in a quantum wire

irradiation A process for semiconductor device fabrication, using irradiation from a focussed ion beam to alter conductive states

junctions Electronic devices developed at FLEET will be formed of two different types of two-dimensional (2D) materials, so we study the processes that happen at junctions between these different 2D materials. Read about an improvement in direct observations of such processes at Monash.

kicked-rotor systems A cycling system ‘kicked’ periodically by additional energy. Think of a freely-rotating bicycle wheel attached to a pedal that receives a literal ‘kick’ every few rotations of the wheel, or a p[endulum poked with a stick every few cycles. A periodically-kicked rotor gas can be realised in the laboratory with atoms periodically kicked by laser pulses.

liquid metal deposition A new method developed at RMIT for depositing 2D materials Read the story.

lithography A technique to ‘write’ some stuff on materials. According to the different microscopic methods used , it can be classified into photo lithography, e-beam lithography, AFM lithography and ion-lithography, etc. See Fan Ji (UNSW)’s lithography microbranding.

low dimensional One-dimensional (1D) systems such as a quantum wire or the edge states of topological insulators, or two-dimensional (2D) systems such as atomically-thin materials

meV milli-electron volts, a unit of energy most commonly used to quantify bandgap. At room temperature, there is approx 25 meV of ambient available, thus a semiconductor must have a bandgap of more than 25 meV, or the gap will ‘close’ due to available energy. (1 meV = the energy acquired by one electron passing through a potential difference of one millivolt.)

microcavities A micrometre-scale structure; an optical medium sandwiched between ultra-reflective mirrors, used to confine light such that it forms exciton–polaritons (in FLEET’s research theme 2).

molecular beam epitaxy (MBE) A method used to deposit thin films of single crystals. Read FLEET studies on the in-line MBE at Berkeley.

monolayer A single 2D layer of material

Moore’s Law A remarkably long-lived phenomenon by which the number of transistors on a unit area has doubled about each 18 months since the 1960s. The semiconductor industry now accepts that Moore’s Law is winding down, with the period extending to three years, and will slow to a halt in the next decades. A related ‘law’, Koomey’s law, explains historical improvements in energy efficiency, which are also winding down.

morphology A fancy way of saying ‘shape’

MOSFET A common transistor technology designed in the 1920s and almost ubiquitous in modern electronics; a type of a field-effect transistor in which there is a thin layer of silicon oxide between the gate and the channel. (Metal–oxide–semiconductor field-effect transistor) A subset of CMOS technology.

multiferroic materials Materials exhibiting more than one ferroic property, eg displaying both magnetic and electronic ordering (see reviewing multiferroics for future low energy data storage)

Na₃Bi A topological Dirac semimetal (see below) studied at FLEET, known as ‘3D graphene’ for its remarkable electronic properties (see study of electronic smoothness).

non-equilibrium state A state temporarily forced by the application of energy, such as light. While the physics of equilibrium states is very well understood, the study of non-equilibrium states has only more recently been researched in such depth. Non-equilibrium physics is key to FLEET’s research theme 3, for example using intense bursts of light at Swinburne.

non-linear interactions Interactions in which forces acting on a system cause disproportionate results

paramagnetic materials Materials that are attracted to magnetic fields and ferromagnets, but which are not magnetised. Materials in which electron spin are randomly distributed in the materials and can only be partially aligned, forming a weak magnetic field.  When an magnetic field is applied, spins have a tendency to align parallel to field direction.

phonon a quasiparticle representing vibrations (or soundwaves) travelling through a material

plasmon A quasiparticle; a quantum of plasma oscillation.

plasmonics The study of surface plasmons, a field of physics at the boundary of physical optics and condensed-matter physics

polariton Hybrid particles: a photon coupled to an electric dipole. This dipole may be an exciton (see exciton-polariton) or oscillating surface electrons (creating a surface-plasmon polariton)

quantum spin Hall effect (QSHE) The spin-orbit interaction driven effect that gives a non-magnetic material conducting edges, which can carry current without resistance, as long as no magnetic disorder is present. First predicted in 2004, QSHE realised the potential for topological materials to carry current with negligible dissipation (at first, QSHE was predicted in graphene, but first demonstrated in the lab in 2007 using HgTe/CdTe quantum wells).

quantum anomalous Hall effect (QAHE) A magnetic version of the QSHE (above), in which conducting edges carry currents in only one direction, and are completely without resistance. First achieved in the laboratory at Tsinghua University in 2013 by Prof Qi-kun Xue (FLEET Partner Investigator at Tsinghua University). While experimental demonstration of QSHE had showed that current could be carried over a distance of microns with negligible dissipation, it was QAHE that showed that current could be carried with no measurable dissipation at all, and it was this 2013 discovery that truly opened up the field of topological electronics now being investigated at FLEET. 

quantum materials include: Topological materials, exciton-polaritons and superfluids, Bose-Einstein condensates, supersolids…

!? Wait, aren’t all materials ‘quantum’? Well yes, technically. However we use the phrase ‘quantum materials’ to refer to materials whose functional properties are dominated by quantum effects. Similarly, all clocks are comprised of atoms, but we use the phrase ‘atomic clocks’ to describe clocks whose function utilises atomic-scale phenomena.

quantum virial expansion The equation of state (ie, the relationship between pressure, temperature and density) of a quantum gas expanded in terms of successively higher powers of the density, which represents the contribution of two-, three-, four-,… body interactions.

quantum wire A narrow constriction linking two hole/electron reservoirs, which are each confined to two-dimensions (ie, they exist in an infinitely flat plane). Because the channel is effectively one-dimensional, charged particles travelling long it experience quantum confinement perpendicular to their path. Effectively, travelling in ‘single file’. And thus the channel’s conductance (ie, how easily those charged particles flow along it) is quantised. Ie, the channel’s conductance increases in ‘step’ jumps.

semi-metal A metal with very small number of electrons compared to normal metal

soliton A quantum particle (or quasiparticle) propagated as a travelling non-dissipative wave

spin A particle’s spin is its intrinsic angular momentum, and is the cause of magnetism. Also see SOI, below.

spin-gapless semiconductors A new class of materials that bridge semiconductors and half-metals, first proposed by FLEET’s Xiaolin Wang. A promising candidate material for spintronics, in which charged particles can be fully spin-polarised.

spin-orbit interaction (SOI) The interaction between electrons’ movement and their inherent angular momentum, which drives topological effects. See experimental verification of SOI at UNSW and study of SOI in GaAs quantum point contacts.

spintronics An emerging field of electronic study in which the ‘spin’ of electrons (their intrinsic angular momentum) is used in addition to their charge.

superconductors A quantum effect in which current can flow without dissipation, but despite decades of research superconductors have remained limited to operate only at extremely low temperatures (and thus are not viable as electronics, because you’d spend more energy cooling the equipment than you’d save!). Inevitably, people confuse superconductivity and superfluid flow…

superfluid A quantum state in which particles flow without encountering any resistance to their motion. Both excitons and exciton–polaritons can flow in a superfluid. Studied in FLEET’s research theme 2.

thermoelectric materials Materials that can convert heat into electrical energy or vice versa, useful for energy harvesting

topological Dirac semi-metal (TDS) Topological material at the boundary between conventional insulators (which don’t conduct) and topological insulators (which conduct along their edges). For example, Na₃Bi (see recent electronic smoothness paper).They are similar to graphene in that electron flow appears massless (electrons moving at a high, constant velocity). TDSs lie at the boundary between conventional insulators (which don’t conduct) and topological insulators (which conduct along edges) so are useful for realising devices in which topological conduction is switched on and off, for example using an electric field.

Alternative (longer) definition: A conventional insulator cannot be converted to a topological insulator without passing through a quantum critical point in which the insulator bandgap goes to zero. At this quantum critical point, the conduction and valence bands touch and the material is a topological Dirac semimetal, possessing a Dirac electronic dispersion around the band crossing. Recently, it was realised that TDSs need not require fine-tuning of parameters to reach a quantum critical point, but may be stabilised by crystal symmetry. Since then, a number of TDSs (Cd3As2, Na3Bi, TlBiSSe, etc.) have been proposed and realised.

topological insulator A relatively new class of material that is electrically insulating in its interior, but conducts along its edges. A subset of topological materials. At increasing levels of mathsyness, see:

• Oleg Sushko’s workshop tutorial
• Michael Fuhrer’s post-Nobel talk
• Michael Fuhrer’s maths-free STEM talk.

topological materials A paradigm shift in material science, first proposed in 1987 and only demonstrated in the lab in the last decade. The importance of topological materials was recognised by the 2016 Nobel Prize in Physics, awarded to Michael Kosterlitz, Duncan Haldane and David Thouless.

topological transistors A proposed class of transistors that would use a topological material as its channel, able to ‘switch’ between conducting (topological insulator, corresponding to ‘open’) and insulating (a trivial insulator, ‘closed’). Topological transistors would form logic circuits just as silicon-based transistors do now.

transistors The tiny switches at the heart of modern computing. Packed a couple of billion into every microchip, they perform the same logic function as components of the 70s eponymous ‘transistor’ radios, or Grampa’s old vacuum valves: put a voltage at the gate wire, and current will flow from A to B; stop the gate voltage, and current stops flowing.

two-dimensional (2D) materials are studied at FLEET for the potential to form ultra-low resistance paths down which current can flow. The most famous ‘2D’ material is graphene, a single sheet of carbon atoms. FLEET studies other 2D materials for their electronic properties. In the case of holes within a transistor, the 2D zone is formed not by the material itself, but by confinement of the charged particles to the interface between materials. Holes are confined at this interface by the voltage between the two semiconductors.

transition metal dichalcogenide (TMD) Atomically-thin materials with useful physical properties for electronic and optoelectronic devices; used as the optical medium in microcavities.

TMDs are a class of van der Waals materials (see below). FLEET studies TMDs of the type MX2, where M is a transition metal atom (molybdenum, tungsten, etc.) and X is a chalcogen atom (sulphur, selenium, or telenium).

Atomically thin monolayers of these TMDs are semiconductors with direct band gaps and other physical properties that are useful for electronic & optoelectronic devices. A 3D material analogous to 2D graphene in that its electrons behave as if they have no mass. These materials lie at the the boundary between conventional insulators (which don’t conduct) and topological insulators (which conduct along their edges) so are useful for devices in which topological conduction can be switched on and off.

van der Waals (vdW) heterostructure A structure made by stacking layers of different van der Waals materials.

van der Waals (vdW) material A material naturally made of 2D layers, held together by weak, electrostatic van der Waals forces, which can be isolated individually or stacked with other materials to form new structures. Layers can be isolated individually or stacked with other materials to form new structures.

wafer scale On a large scale, physical size large enough for commercial chip production, cf lab scales. There are some limitations on growth of some materials to this scale, and it’s the point at which we say “we’re done!” In industry context, it would mean 100mm diameter semiconductor chip… but in the lab, it can mean 1in size.

Weyl elemental particle (theoretical) has spin direction parallel to antiparallel to its direction of motion, and has no mass.

Weyl semi-metals Similar to a topological Dirac semimetal, but with unusual surface states that may lead to dissipationless conduction. A topological materials that can theoretically carry electrical charge without dissipation. In Weyl semi-metals, the (theoretical) Weyl quasipartical (massless,) can carry current relativistically (the same as in graphene), ie without dissipation of energy. and electron spin can either parallel or antiparallel to its motion direction. The metallic property in a Weyl metal is very stable and can not be destroyed by any external influence. Also see semi-metal.

Xene Topological insulators based on group IV and group V materials (the ‘groups’ refer to the periodic table)